Present day land line telephone systems such as PSTN (Public Switched Telephone Networks) in the USA convert a voiceband channel to a digital stream referred to as a DS0 level having a bit rate of 0.064 Mb/s (Megabits per second). The DS0 level comprises 8 bit bytes representing a voice level at any given instant in the conversion process. Traditionally these 8 bit bytes are referred to in the art as octets. A plurality of these DS0 channels are stacked (multiplexed) onto higher level bit streams. An example of a higher level bit stream is a DS1 level containing 24 DS0 or voice-grade channels. In the United States, a DS1 channel operates at 1.544 Mb/s. Since 24 multiplied by 0.064 Mb/s is 1.536 Mb/s it is apparent that this leaves 8 kb/s (kilo bits per second) for overhead bits to be used for framing or synchronization purposes. A frame includes an octet of each of the 24 channels in a DS1 bit stream. The first octet of the frame represents the voice level in channel 1 and the last octet represents the voice level of channel 24.
The framing bits are used at the receiving end of a transmission line or immediately prior to a switch to define the beginning of a frame or in other words where channel 1 starts. In telephony parlance, a series of 24 channels of data bits is referred to as a frame and 12 frames are referred to as a superframe in one frame detection format used in the United States. When using the 12 frame superframe (SF) format or the 24 frame extended superframe (ESF) format, the least significant bit of each voice channel is "robbed" every 6th frame and replaced with signalling information associated with the voice channel. This voice channel associated signalling information is used for channel supervision and addressing, such as call setup and call completion.
These "robbed" bits are not discernable by a user when voice communication occurs. However, the technique may cause errors when binary data is transferred via modems over a voice grade channel unless special precautions are taken to determine the time when the robbed bits occur and synchronize the data transmission accordingly.
In the prior art, a software entity, or object designated as an interworking function (IWF-basically a modem function) has been used in CDMA (Code Division Multiple Access) type cellular communication systems to convert data to a protocol and format compatible with the destination network receiving the data. As is known to those skilled in the art, CDMA systems provide direct digital transmission in a low-bit-rate speech coding scheme that does not allow transmission of voiceband modem signals.
In a CDMA system, the circuit mode data services utilize the direct access to the digital radio channels. Information is exchanged as packetized data between the mobile terminal and the IWF using established data protocols in conjunction with the digital radio channel. At the IWF, the data packets received from the mobile terminal are converted into voiceband modem signals suitable for transmission over the PSTN (Public Switched Telephone Network) and vice versa. Thus the CDMA circuit mode data services have been used for point to point connectivity through the PSTN wherein a mobile terminal such as a portable PC (personal computer) may be interconnected to another unit such as a wireline desktop PC running in either the asynchronous data reception or in the fax application mode and data may be transmitted therebetween.
When a voice call is made from a MS (mobile station) to a PSTN termination point, the call passes through a BTS (Base station Transceiver Subsystem) to a BSC (Base Station Controller) and then to an MTX (Mobile Telephone eXchange) before being passed to the PSTN network. The voice call uses various software entities within the BTS, the BSC and the MTX including a SOE (Service Option Element) and a DSP (Digital Signal Processor) and outputs a PCM (Pulse Code Modulated) DS0 signal. A data call, in one prior art embodiment, used the same entities except that the software load for the SOE and DSP was different and the DS0 signal output from the DSP was in a ISLP (Inter-System Link Protocol) format. The data signal is passed through the IWF to alter the protocol to a PSTN compatible format before being returned to an MTX for forwarding to the PSTN. The prior art systems referenced used an out of band ethernet interface for exchange of control and signaling messages between the MTX and the IWF for these data calls. This design required an IWF entity for each MTX providing data call service. Especially in new systems, this can be a considerable waste of resources.
It would be more desirable to be able to use a single IWF for a plurality of MTXs in a system with the option of adding IWFs as the number of data calls is noted to be increasing.
Prior art mobile stations only had the capability of transmitting data on one RF channel due to system and MS constraints. These constraints resulted in a maximum data rate transfer of 9.6 Kbs. In such a system, a BSC transports data received over one RF channel from an MS to the IWF, over one DS0 channel between the BSC and the IWF. Thus the design of an MTX, its BSC and the interconnection from the BSC through the MTX allowed a maximum data rate of only 9.6 Kps over one DS0 channel. Presently proposed new equipment standards permit up to at least 8 RF channels to be used simultaneously by a mobile station for transmitting a parallel burst of data packets, one on each of several RF channels, to or from an associated BTS thus accomplishing a total quantity of data transmitted to be at a rate higher than the maximum transmission rate possible with one DS0 link. Networks on the PSTN side of the communication system are also now capable of receiving or transmitting data on more than one DS0 channel simultaneously. Thus, it would be desirable that the MTX and associated BSC equipment have the capability to interconnect the new proposed mobile stations and similar equipment connected to wireline facilities for high speed data transfer with minimum redesign.